Additional professional research into the crop circle phenomenon is relatively sparse -- partially, we suspect, because the mass media attributes crop circles to "pranks with planks" (manual flattening of the crops by people) which has resulted in there being little funding available to support serious scientific inquiry. We list below a few links to other work which has been carried out which may be of interest.

AQUIFER RESEARCH:
"The Underground Connection" is interesting look at the
relationship between the underground water-table and the placement
of crop circles in England during the years 1993-1998. This research
supports a later discovery by John Burke that ground electrical charge
in Wiltshire County (where a majority of the British crop circles
occur each summer) increases slightly over the summer, as the water-table
lowers. Such an increase in ground electrical charge may function
as an "attractor" to W.C. Levengood's hypothesized plasma
vortex energy system as the causative mechanism behind the phenomenon.http://www.sacredbritain.com/research-underground.html

H-GLAZE REPORT: In 1993, during the Perseid meteor
shower in August, a crop formation occurred in Wiltshire, at Cherhill.
This formation was remarkable primarily because of a "metallic
glaze" that was observed on the plants in some of the swirled
sections of the formation. This material was sampled by the first
visitors to the formation and some of these samples were sent immediately
to W.C. Levengood and John Burke in the U.S. for analysis. Levengood
and Burke's peer-reviewed paper, "Semi-Molten Meteoric
Iron Associated with a Crop Formation"
was published in 1995 in the J. Sci. Exploration. http://www.bltresearch.com/semi-molten.php

In 2001 Rodney Ashby, an Englishman interested in the crop circle
phenomenon, posted a report on his web-site entitled "The
H-Glaze Report," which presented a detailed account
of his examination of samples he had received in 2001 (8 yrs. after
the original event) – materials purportedly recovered from the
same 1993 Cherhill, UK crop formation. Mr. Ashby's original 2001 report
supported many of the findings presented in the Levengood/Burke paper,
in particular the evidence pointing to intense heat having been involved. For reasons known only to Mr. Ashby, he has now (2010) removed his original "The H-Glaze Report" paper which supported the Levengood/Burke research--it is no longer present anywhere on his web-site. Anyone interested in reading this original paper should contact Nancy Talbott in the BLT office.

In early 2005 Mr. Ashby posted a second report, "The
H-Glaze Explained," in which he states that he has examined
a material "recently" supplied to him (by a self-proclaimed
crop circle hoaxer) -- which material he now believes to be the source
of the magnetic material discussed in both the Levengood/Burke 1995
paper and his own original H-Glaze report.http://www.xstreamscience.org/Old_H_Glaze/h_glaze_0.htm

Because there were inconsistencies in Mr. Ashby's two reports, as
well as a number of facts presented in the Levengood/Burke paper which
Mr. Ashby did not address at all, Levengood and Burke have issued
their "Comments" on Ashby's 2005 posting,
which can be read at the following web-site:www.kornkreise-forschung.de/textHglazeNew.htm.

COMMENTS on Levengood, W.C. and Talbott, N.P. (1999) "Dispersion
of Energies in Worldwide Crop Formations," Physiol. Plant.105:615-624.
Remarks by Eltjo H. Haselhoff, Ph.D, in Letters to the Editors.
For copies of Dr. Haselhoff's remarks contact him at dcccs@planet.nl

METEOROLOGICAL INTERPRETATION: British meteorologist George
Terence Meaden spent considerable time investigating crop circles
in the 1980s and came to the conclusion that "simple" circles were
caused by local wind vortices and that all others were man-made. He
authored several books which are now, unfortunately, out of print,
but he has posted a summary of his opinion on the web and can be contacted
for further comment.http://www.stonehenge-avebury.net/scienceofcropcircles.htm

SHORT-LIVED RADIONUCLIDES: A 1991 study published by Marshall Dudley and Michael Chorost reported the discovery of 13 short-lived radionuclides in soil samples from one British crop circle. However, a second paper by these authors in 1992 presented the "entirely negative" results of their subsequent work and retracted the 1991 paper's conclusions:

"We were much more organized [in 1992] and had really good
equipment onsite. It was sensitive enough to detect the radiation
from the potassium in a banana. But apart from natural background radiation and some cesium-137 from Chernobyl, which
had blown six years before, we found nothing significant."

BLT did not know about the 1992 follow-up paper, but had posted the link to the 1991 paper. In 2012 Michael Chorost requested that we remove the link to the inaccurate 1991 study, saying he wanted to get the paper off the internet "because it's just plain wrong." For any further information contact Michael Chorost in Washington, D.C. (http://www.michaelchorost.com) and/or Marshall Dudley in Oak Ridge, TN.
GEOMETRY & CROP CIRCLES: Many individuals interested in crop circles have been intrigued by the complex geometry inherent in a number of formations. How such precision is accomplished on undulating and/or sloping fields, covering large areas of ground, in the dark of night and in all types of weather, stimulates real curiosity. The fact that many of the standing-crop elements found inside crop circle perimeters could have only been referenced from external points in the cropfield -- where the crop is found to be untouched -- adds to the mystery. There are many web-sties which investigate this aspect of the circles; here are a few:Gerald S. Hawkins,
Ph.D, D.Sc, former Chairman of the Dept. of Astronomy, Boston University,
has found that some crop circle patterns embody geometric theorems
and diatonic ratios (the simple whole-number ratios which determine
a scale of musical notes).http://www.lovely.clara.net/hawkins.html

Hawkins also discovered a fifth, more general, Euclidean theorem,
from which he could derive the other four. However he could find no
reference to this fifth theorem in the works of Euclid or elsewhere,
and he subsequently challenged the readers of both Science News
and The Mathematics Teacher to come up with this unpublished
theorem. See the article published in Science News (Oct. 12, 1996)
at:http://www.sciencenews.org/sn_arch/10_12_96/note1.htm

Bert Jansen, also from The Netherlands, is a producer, photographer
and award-winning videographer in the crop circle field who is also
writing a booklet on the geometry found in crop circles. (Click on
"Crop Circles" on left-hand side, then scroll down to "Crop Circle
Reconstructions" & "Crop Circle Geometry.")http://www.bertjanssen.nl
(scan down to "Crop Circle Geometry")